1. Field of the Invention
The present invention relates to optical telecommunication systems and, in particular, to an apparatus and method of manufacturing optical devices employed in such telecommunication systems.
2. Technical Background
Up to three port filtering and isolating packages are widely used in local and long distance optical telecommunication networks. These networks comprise various spectral shaping and isolating optical assemblies as parts of dense wavelength division multiplexing (DWDM) systems. The necessity to design reliable optical devices for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime, is of significant importance. A typical example of such optical devices is an optical filter assembly. A typical optical filter assembly comprises two (input and reflective) optical glass fibers inserted into a dual-capillary ferrule to produce a fiber-ferrule sub-assembly, a GRIN lens, and a filter. The optical components of the filter assembly are embedded into an insulating glass tube, which in turn is mechanically protected by a metal housing. In a typical 3-port package the above dual-fiber filtering assembly is combined with an output collimating assembly leading to a single optical fiber. These filter assemblies typically exhibit insertion losses higher than desired, resulting in degraded overall performance of the communications system or module. The problem is particularly acute during exposure to ambient operating conditions where temperature is variable.
Typical input glass ferrules employ one of two designs. A single capillary suitable for containing multiple glass fibers or separate circular capillaries for each fiber have been used, each with relatively short (0.7-1.2 mm) fiber-receiving conical lead-in ends. With such input ferrules, the optical fiber is subjected to an S-bending over the short conical end portion which typically exceeds 50% of the fiber diameter (for a fiber having a 125 μm diameter) on a span of about 6 to 10 diameters in length. This excessive micro bending increases the insertion losses. Although the multi-capillary design reduces the lateral deflection of fiber interconnects compared to the elliptical single-capillary design, the short length of the cone end of such ferrules cannot reduce the micro bending of the fiber and its inherent insertion loss. Fiber-ferrule subassemblies employing such ferrules are manufactured by inserting the optical fibers stripped of their polymer coating into the respective ferrule capillaries; epoxy bonding the fibers into the ferrule capillaries, including the conical end portions; grinding and polishing an angled facet on the fiber-ferrule; and depositing on the polished surface an anti-reflection (AR) coating. Once finished, the fiber-ferrule is aligned and assembled with the collimating GRIN lens and then embedded into the insulating glass tube, which, in turn, is protected by a metal housing.
There are two different technical solutions used in the design of bonds securing the components of an optical assembly. A low compliance bond between thermally well matched glass fibers and the glass ferrule is an approach commonly used by some manufacturers. The adhesives used are heat-curable epoxies with high Young's modulus (E>100,000 psi) and moderate to high thermal expansion coefficients (α=40-60 10−6 ° C.−1). A typical example would be 353 ND EPO-TEK epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched glass tubes with metal housings and glass optical elements with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150° C. in the presence of moisture, are typically characterized by an extremely low Young's modulus (E<500 psi) and high thermal expansion (α=180-250 10−6 ° C.−1). A typical example would be DC 577 silicone, which can be used to bond, for example, a metal optical filter holder to a GRIN lens.
Adhesive bonding with subsequent soldering or welding is used to encapsulate a filtering assembly into a three-port package of a DWDM module. A precise alignment achieved during initial assembly of a filter prior to final packaging can be easily decreased due to the adhesive curing process and the high temperature thermal cycles associated with soldering or welding during the final packaging of the components. Such manufacturing processes and resulting components have several problems resulting from stresses on the optical components due to the thermal contraction mismatch between the glass and metal materials, polymerization shrinkage in adhesive bonds, and structural constraints induced by bonding and final soldering during encapsulation. These stresses lead to displacements of optical components during bonding and soldering, resulting in 0.3 to 1 dB or greater increases in the insertion loss.
Such a filter package enclosure, which is typically formed of six to eight concentric protective units, has micron transverse tolerances. Maintaining these tolerances requires precision machining, time-consuming alignment, and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering typically includes several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive, resulting in repositioning of optical components and a shift in the filter spectral performance. With such design, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
However, for many applications, it is desirable to obtain a high accuracy, thermally compensated optical multiple-port package that can be relatively inexpensive, reliable, and have a low insertion loss. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components, but also to compensate for and minimize the thermally induced shift in spectral performance. Further, it is desirable to obtain a multiple-port package, such as six port packages, with the same qualities since they further reduce costs, reduce size, and also result in reduced insertion loss. Thus, there exists a need for such optical packages and a process for manufacturing such optical packages, which is miniaturized, has a low insertion loss, is inexpensive to manufacture, and which results in a device having reliable, long-term operation.